Fig. 1 mer structures of PEG, PEO, PVA, PAA and PMAA [11]Fig. 2 Hydrolysis of PVAC to produce PVA [13]Fig. 3 Hydrogen bonding in commercial PVA (a) at high hydrolysis many secondary hydrogen bonds can be established. (b) at low hydrolysis, acetate groups act as spacers and restrict the level of hydrogen bonding. [14]Fig. 4 Schematic diagram of the interrelationship between apparent viscosity and degree of hydrolysis, and between solubility and degree of hydrolysis for aqueous PVA solution [12].Fig. 5 (a) Schematic illustration of the structure of monoclinic lattice; (b) Crystal structure of PVA. PVA chains are projected on the (101) plane. The circles in descending order of size represent oxygen, carbon and hydrogen atoms, respectively. The dashed and solid circles distinguish between hydrogen atoms on opposite sides of the chains [19].Fig. 6 Density of PVA as a function of crystallinity. Data are shown for Mw =14000, 31000, 57000, 10000, and 20000 g/mol. [15]Fig. 7 Solubility of PVA in water as a function of temperature. Data for various grades of PVA are shown. A, 78–81 mol% hydrolyzed, DP = 2000–2100; B, 87–89 mol% hydrolyzed, DP = 500–600; C, 98–99 mol% hydrolyzed, DP = 500–600; D, 98–99 mol% hydrolyzed, DP = 1700–1800 [17].Fig. 8 Schematic illustration of the dissolution of PVA as a function of time [20].Fig. 9 Solution viscosity of PVA as a function of temperature. A, DP=2200; B, DP=1500; C, DP=550; D, DP= 220. (Concentration = 16 wt %, 87-89% hydrolyzed)[17].Fig. 10 Solution viscosity at 60°C as a function of concentration. Data for various grades of PVA are shown. Information on the different grades of PVA used in this investigation are given in III [13].Fig. 11 Typical plot of the Mark-Houwink equation for an aqueous PVA solution [22].

VFig. 12 Surface tension of aqueous PVA solutions 20°C as a function of concentration. The degree of polymerization in the PVA was 1700. A, 98-99% hydrolyzed; B, 87-89% hydrolyzed; C, 78-81% hydrolyzed [17].Fig. 13 Surface tension of aqueous PVA solutions 20°C as a function of concentration. The degree of hydrolysis in the polymer was 87-89 mol%. A, DP = 1700; B, DP = 550. [17].Fig. 14 Effect of NaCl additions to aqueous PVA (Mw=72,000 g/mol) on the surface tension of the solution at 30°C [25].Fig. 15 Effect of salt concentration upon apparent viscosity for a 10% PVA, Mw=100000, 88% hydrolyzed aqueous solution, T=25°C, shear rate=46/s [12].Fig. 16 Tensile strength as a function of relative humidity for fully hydrolyzed poly(vinyl alcohol) films. A, Degree of polymerization=2400; B, 1700; C, 500 [11].Fig. 17 Photograph of a porous PLGA scaffold used for tissue engineering. The porosity was induced by a porogen, sodium chloride of size range 300-500μm [1].Fig. 18 Schematic illustration of electrospinning and electrospray processes [31]Fig. 19 Schematic illustration of the set-up for producing 3-D structures [34].Fig. 20 Surface area in the porous structure as a function of fiber diameter for various processing techniques [35].Fig. 21 A schematic illustration of the various physical phenomena occurred during electrospinning a viscoelastic polymer [43].Fig. 22 Various instabilities that may be induced in the viscoelastic jet that is ejected from the Taylor’s cone [43].Fig. 23 Photographs showing round [44] (a) and flat (b) [47] fibers in electrospun PEO.Fig. 24 Photographs showing branching (a) and splitting in electrospun HEMA [47].Fig. 25 Photographs showing bead-on-string morphology in the electrospun polymer [48].Fig. 26 Mesh-like structure in electrospun EVOH [34].Fig. 27 Schematic illustration of the effects of process parameters on the the structure of the electrospun product [37].

VIFig. 28 Photographs showing the structure in electrospun PEO (a) solution conductivity 1.23 Coulomb/liter (b) solution conductivity 28.2 Coulomb/liter [48].Fig. 29 Schematic of the experimental set-up. Samples for microscopic examination were obtained from the center(X) of the deposition area. The diameter of the deposition area was generally on the order of 2 cm in most experiments.Fig. 30 Variation of solution viscosity with molecular weight and concentration. The measured viscosity data from the literature has been fitted to equation (6) [13]. This equation was then used to predict the viscosity for molecular weights and concentrations used in this study. The letters in the legend correspond to the molecular weight information shown in Table IX.Fig. 31 Variation of dimensionless concentration [η]c with the concentration of PVA in aqueous solutions. Data have been plotted for experimental conditions under which stable fiber structures were produced. The intrinsic viscosity has been calculated from the Mark-Howink equation.Fig. 32 Examples of bead on string structures in the electrospun polymer. Such structures were typically observed at low Mw and concentration (a) Mw = 9000- 10000, C = 22 wt % and (b) Mw =50000-85000 g/mol, C = 9 wt %.Fig. 33 Examples of fibrous structures with round fibers. (a) Mw = 9000-10000 g/mol, C = 22 wt % and (b) Mw =50000-85000 g/mol, C = 15 wt %.Fig. 34 Examples of fibrous structures with flat fibers. (a) Mw = 124000-186000 g/mol, C = 8 wt % and (b) Mw =31000-50000 g/mol, C = 22 wt %.Fig. 35 Examples of coiling and bending (a) and extensive elongational flow (b) in fibers.Fig. 36 Examples of branching. Note the secondary branching in (b).Fig. 37 Examples of fiber splitting. (a) Splitting into two sub-fibers from a bunch of merged fibers; (b) Splitting into two sub-fibers from a single fibers (c) Spliting into three sub-fibers, two of which are thinner and travel in the direction of the primary fiber, and the other one is similar in diameter with the primary fiber but travels at an angle of around 45º with the direction of the primary fiber.Fig. 38 Photographs showing the breakdown of a fully formed jet for two different values of [η]c. (a) 6.5 (b) 10.

VIIFig. 39 Sequential photographs showing the nature of the solution jet for various times (s) after the application of the voltage. The voltage was applied at t = 0 s. ([η]c = 6.5)Fig. 40 Sequential photographs showing the nature of the solution jet for various times (s) after the application of the voltage. The voltage was applied at t = 0 s. ([η]c = 10)Fig. 41 Photographs illustrating the position of a minijet in successive frames. By monitoring the position of a minijet in successive frames, the local jet velocity was calculated.Fig. 42 Average jet velocity as a function of [η]c. The velocity values for before (Y) and after (X) the application of the voltage are shown.Fig. 43 Photographs showing the effect of concentration (wt %) for two different values of Mw.Fig. 44 Photographs showing the effect of concentration at a constant concentration (9 wt %) (a) Mw = 50000-85000 g/mol; (b) Mw = 124000-1860000 g/molFig. 45 Distribution of fibers at a constant concentration (9 wt %) (a) Mw = 50000- 85000 g/mol (b) Mw = 124000-1860000 g/molFig. 46 Variation of average diameter with molecular weight and concentration.Fig. 47 Photographs showing the changes in the structure with increasing values of [η]c.Fig. 48 Variation of average fiber diameter with dimensionless concentration [η]c. The critical [η]c values for transition from extremely dilute to dilute to highly entangled regions are also indicated [62].Fig. 49 Fiber distribution of (a) [η]c=4.6 (Mw=9000-10000 g/mol, C=22 wt %); (b) [η]c=21.8 (Mw=89000-980000 g/mol, c=16 wt%).Fig. 50 Variation of the aspect ratio with [η]c for various molecular weights.Fig. 51 Types of distributions in the fibers for various molecular weights and concentrations. The [η]c values are also indicated.

VIIIFig. 52 Jet breakdown of a Newtonian fluid at high Reynolds number (or low η). Note the formation of drops and satellite drops. Each drop can break down further into smaller drops and satellite drops.Fig. 53 Jet break-up in a Newtonian fluid at low Reynolds number (or high η) [60].Fig. 54 Deformation, necking and breakup of a highly viscous Newtonian drop of fluid.Fig. 55 Schematic illustration of the breakdown of viscoelastic systems. (a) Stepwise repeated breakup at Cacrit. (b) Affine stretching of drop into a thin liquid thread at Ca >> Cacrit and eventual disintegration into droplets.Fig. 56 Schematic illustration of the jet breakup for various types of fluids. The important dimensionless numbers are also indicated.Fig. 57 Variation of average fiber diameter with the Ohnesorge number. The inserts show the wavy fibers at low Oh and straight fibers at high Oh.Fig. 58 Variation of initial Oh with [η]c for various molecular weights.Fig. 59 Variation of initial (t = 0) and final (t = large) Oh with [η]c. The Oh varies during the process as the jet diameter decreases.Fig. 60 Variation of initial (t = 0) and final (t = large) Oh with [η]c for various molecular weights. The Oh varies during the process as the jet diameter decreases. The letters in the legend correspond to the data shown in Table IX.Fig. 61 Weight loss as a function of time under ambient conditions.Fig. 62 Photographs showing the effect of NaCl on electrospun PVA (a) 0% (b) 0.5% (c) 1% and (d) 3%. (Mw = 9000-10000 g/mol, c=23 wt %).Fig. 63 Photograph showing the presence of salt crystals on the bead. Such precipitation of salt was observed throughout the sample.Fig. 64 Photographs showing the effects of NaCl additions to PVA (a) 0% (b) 1% (Mw = 124000-186000 g/mol, c=7 wt %).Fig. 65 Photographs showing the effects of polyethylene glycol additions to PVA (a) 0% (b) 5% (c) 10%.Fig. 66 Distribution of fiber diameters in electrospun PVA with (a) 5 wt% PEG and (b) 10 wt% PEG.

IX LIST OF TABLES

Table I List of biopolymers used in tissue engineering and drug delivery applications [5]Table II Typical Properties of common Biodegradable Polymers [4]Table III Degree of polymerization and %hydrolysis for the various grades of PVA in Fig. 10.Table IV Surface tension of solutions containing various amounts of PVA [17].Table V Advantages and Disadvantages of various processes currently used to produce porous polymers [29]Table VI Varicous factors associated with electrospinning of polymers from solution [43].Table VII Weight average molecular weight (Mw) and % hydrolyzation of PVA used in this study.Table VIII Relevant properties of the solvents used in this study [54-56].Table IX Summary of concentrations used for each molecular weight. Only those concentrations at which a fibrous structure could be obtained was selected for each molecular weight. The solvent was distilled water at 80°C.Table X Summary of conditions used to produce porous polymers with solvents other than water.Table XI Mark-Houwink constants for PVA solutions obtained from various sources in the literature.Table XII Variation of Ca, Re and Oh numbers for various conditions. The corresponding distribution of the fiber diameters is also shown.

X1. INTRODUCTION

Porous polymeric structures are used in a wide range of applications including wound

samples [18]. The density of PVA varies from 1.19 g/cm3 for completely amorphous

sample to 1.31 g/cm3 for completely crystalline sample while generally it will be found to

lie within the limits of 1.28 to 1.31 g/cm3 [17,18]. The crystallinity of PVA tends to

decrease with increasing molecular weight and decreasing hydrolysis. Long molecular

chains involve impeded segmental motion and thus make it more difficult for the

molecules to fold up into crystalline structures. Hydrolysis decreased with increasing the

number of residual acetate group in the molecules. The bulky size of the pendent acetate

group prevents the molecular chains to closely fold up to form crystalline.

11(a) (b)

Fig. 5 (a) Schematic illustration of the structure of monoclinic lattice; (b) Crystal structure of PVA. PVA chains are projected on the (101) plane. The circles in descending order of size represent oxygen, carbon and hydrogen atoms, respectively. The dashed and solid circles distinguish between hydrogen atoms on opposite sides of the chains [19].

Fig. 6 Density of PVA as a function of crystallinity. Data are shown for Mw =14000, 31000, 57000, 10000, and 20000 g/mol. [15]

122.2.3 Solution behavior of PVA

The solubility, viscosity, and surface tension of PVA depend on temperature,

concentration, % hydrolysis and molecular weight of the material. PVA is soluble in

were obtained from the center(X) of the deposition area. The diameter of the deposition area was generally on the order of 2 cm in most experiments.

47Table IX Summary of concentrations used for each molecular weight. Only those concentrations at which a fibrous structure could be obtained was selected for each molecular weight. The solvent was distilled water at 80°C.

Solvent T ( ºC) Mw (g/mol) Concentration (wt %)

The breakup of polymer jets into droplets and fibers is strongly influenced by rheological

properties of the solution. High molecular weight polymers added to solutions may

suppress the breakup and atomization of the solution. In many commercial applications,

macromolecules are intentionally added to control misting or suppress the formation of

droplets less than 5 µm [57]. For example, high molecular weight polyisobutylene is

intentionally added to machining fluids and jet fuels to prevent spray formation. It may

also be added in spray paints to increase the overall drop size. Numerous studies have

shown that the breakdown of solutions containing polymers is strongly influenced by the

rheological properties of the solution [58]. Salient aspects of solution rheology with

respect to PVA solutions are discussed in the following sections.

5.1 Viscosity of PVA solutions

The viscosity of PVA solutions (η) depends on the molecular weight (Mw), concentration

(c), degree of hydrolysis and the type of solvent. The dependence of zero shear viscosity

on Mw in many polymers can be described by the following Power law equation [59]:

η = K ' (M w )3.4 (8)

In general, the effects of Mw and c on solution viscosity can be modeled as [59]:

η = K (cρ )α (M w )β (9)

50The measured viscosity data for PVA solutions in water [13] has been fitted to the above

power law equation. The exponents α and β were calculated to be 4.39 and 2.90

respectively. Equation (9) was then used to generate viscosity data for the molecular

weights and concentrations used in this study as shown in Fig. 30. Note that the viscosity

of the solution depends strongly on Mw and concentration. The intrinsic viscosity [η] for

polymer solutions can be related to Mw by the Mark-Houwink equation:

[η ] = K '' (M w ) a (10)The Mark-Houwink constants (K” and a) for PVA reported in the literature for various

conditions are summarized in Table XI. The product of [η] and c can be used to define

10 F E D C

8 Viscosity (Pa s)

2 B

A 0 0 0.1 0.2 0.3 0.4 ConcentrationFig. 30 Variation of solution viscosity with molecular weight and concentration. The measured viscosity data from the literature has been fitted to equation (6) [13]. This equation was then used to predict the viscosity for molecular weights and concentrations used in this study. The letters in the legend correspond to the molecular weight information shown in Table IX.

51[η]c, a dimensionless concentration. The typical variation of [η]c with c for the values

of Mw and c at which stable fibrous structures were obtained is plotted in Fig. 31. As can

be expected, [η]c increases with c for various molecular weights. As Mw increases, the

slope of lines in Fig. 31 increases. This result suggests that Mw has a greater effect on the

rheological properties of the solution than the concentration. The viscoelastic behavior of

polymer solutions can be divided into various regions depending on the value of [η]c. In

dilute solutions, [η]c < 1 and the viscosity does not change much with concentration.

The entanglements become significant for [η]c > 4. For [η]c > 4, the viscosity begins to

Table XI Mark-Houwink constants for PVA solutions obtained from various sources in the literature.

(a) (b) (c)

Fig. 36 Examples of branching. Note the secondary branching in (b).

56 (a) (b) (c)Fig. 37 Examples of fiber splitting. (a) Splitting into two sub-fibers from a bunch of merged fibers; (b) Splitting into two sub-fibers from a single fibers (c) Spliting into three sub-fibers, two of which are thinner and travel in the direction of the primary fiber, and the other one is similar in diameter with the primary fiber but travels at an angle of around 45º with the direction of the primary fiber.

The fibers may exhibit branching as shown in Fig. 36. Branched fibers ejecting almost

perpendicular from the surface of the primary fiber were found for both straight and

coiled fibers. The branched fibers tend to taper away within a short distance (Fig. 36 (a))

Fig. 60 Variation of initial (t = 0) and final (t = large) Oh with [η]c for various molecular weights. The Oh varies during the process as the jet diameter decreases. The letters in the legend correspond to the data shown in Table IX.

79The large surface stress stabilizes thin filaments and resists further fiber breakdown. The

data shown in Fig. 60 indicate that for different values of [η]c, at a limiting Oh number

the surface stresses become so large that splaying essentially stops as the fiber is

stabilized.

5.7 Effects of Solvents

A variety of solvents can be used for producing PVA solutions. These include water,

DMSO, NMP and ethylene Glycol [20]. Although water has been used extensively as a

solvent, it is only a moderately good solvent for PVA [20]. Because of aggregation and

micro-gelling, it is difficult to obtain uniformly dispersed molecular solutions. As a

result, dissolution of the PVA is rather difficult. The temperature has to be increased to

80°C to achieve dissolution. The solution viscosity and [η] can also change with the type

of solvent. In addition, the rate of evaporation can be different for various solvents. All

these factors can influence the electrospinning process.

Attempts to produce fibers with other solvents were not very successful. Although a

visible jet was detected with all the solvents, the polymer on the collector tended to

agglomerate when DMSO, EG or NMP was used as a solvent. This agglomeration was

primarily due to the lack of adequate solvent evaporation as shown in Fig. 61. The

boiling points and the heats of vaporization of DMSO and EG are much higher than

water. EG absorbs twice its weight of water at 100% relative humidity [11]. DMSO is